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Scuds and Patriots: Timing is Everything

.3433 seconds. A third of a second isn’t very long. I try to think of things I can do in that amount of time, but it’s pretty hard. I could maybe blink my eyes, or swivel my head, or count the cash in my wallet. But not much else comes to mind, which must mean I can’t think very quickly, either.

But search times for computer hard drives are measured in milliseconds, or .00000x seconds. Computer memory speeds are measured in nanoseconds, which is .00000000x seconds (that’s super fast). In the computer world of circuits and gates and discrete logic, .3433 seconds is an eternity.

.3433 seconds in a computer can get people killed…it has gotten people killed, and it’s the subject of Today’s History Lesson.

In February of 1991, eyes all over the world were glued to their TV’s, and the TV’s were glued to the Middle East, as massive Coalition military forces were expelling the Iraqi military from Kuwait. We watched in wonder as TV-guided bombs gave us front-row seats as they plowed into buildings. Tomahawk Cruise missiles lauched from planes, ships, and subs traveled hundreds of miles and blew up designated targets with impugnity. And then came the 100-hour ground campaign, where Coalition ground forces swept in from the west, bludgeoning every enemy in its path.

It’s here that our focus narrows. During this 100 hours, the fear of Scud missile launches from Iraq was at its greatest. The small possibility that they would be loaded with chemical agents targeted at either Saudi Arabia or Israel kept everyone on high alert, including the Patriot missile batteries that were used as missile defense. Nobody dared shut anything off, and for the Patriot, that caused a problem.

The Patriot missile used probability to interrogate target value and the path to that target. If a target object was in one place at one moment and then traveled a certain distance in the next, the system determined it to be hostile and projected a “path of interception.” But since ballistic missiles (like the Scud) travel at very high speeds, and Patriots traveled very quickly, the closure speeds were measured in the thousands of miles per hour. So that intercept and detonation window was extremely small, which is fine and good.

But the Patriot’s memory constraints had caused the programmers to “round off” the clock value, and then use that slightly inaccurate value in the next calculation. So over time, the Patriot got less accurate. Again, this wasn’t a problem by itself, because the Patriot battery was a mobile system that was designed to be put in place, run for several hours (or a day), and then moved. Shutting down the system for transport would clear the inaccuracy.

But these Patriots, with the heightened concerns, were not shut off. And by February 25, 1991, they had been running for more than 4 days. So when a Scud was fired towards Dhahran, Saudi Arabia that evening, the triggering device in the Patriot was off by .3433 seconds. And the immense closure speeds of the two missiles meant that the Patriot detonated more than 2,000 feet below the descending Scud. The shockwaves were enough to damage the incoming Scud, but not destroy it. Scuds were not sophisticated enough to be precision-guided, but they could be area-targeted, like on a city. This Scud hit a random target (like all the rest), but that happened to be a U.S. military barracks, and it killed 28 U.S. Reservists.

The next day, a package arrived in Dhahran. In a tragic coincidence, Israeli Patriot operators had discovered the timing error on February 11th and notified the authorities. Immediately, Raytheon (the missile designer) set its programmers to work and, on the 16th, sent out a software “patch” that addressed the problem. The software for the Dhahran batteries arrived on the 26th.